[Martyc]

I have a 1/2 inch thick rectangular piece of white HDPE that has 4 slots that are 1/4 inch wide by 3 1/2 inches running across the width of the piece. I want to round over the edges of the slots. Any suggestions on how to proceed?

Thanks

 

[Stick486]

treat it like wood...

 

[TWheels]

[
Plastics like HDPE can be worked as if they were wood, as @Stick486 said. However, the feed rate must be much higher in order to avoid melting. If you have scrap try that first. If I were doing what I think you want to do, I would use the following router bit:
https://www.freudtools.com/explore/router-cnc/edge-treatment/ovolo

 

[1fizgig]

Couldn't you treat it a bit like a mortise, but using a bit that'll chamfer both sides and the ends in one pass? Might require a fiddly set up, but it was just a random thought off the top of my head.

Might reduce the amount of heat building up in the bit so it's less likely to melt the plastic as yuo go?

 

[ger21]

Use a point cutting roundover bit, with a template and guide bushing.

Melting should not be an issue at all when cutting roundovers.

 

[gdonham1]

As other have pointed out you have to move right along or you will melt. Do not let the router bit sit too long in one spot. Use a plunge router if you have one and if you have to stall out then lift the router bit up and out. So make a dry run of what you are going to do and keep your router cord out of the way. If you are going to use a jig to guide the router make sure you can make a smooth pass and raise the bit at the ends. If you need to back in to clean up an edge you can do that but keep it moving along.

 

[TWheels]

Use a point cutting roundover bit, with a template and guide bushing.

Melting should not be an issue at all when cutting roundovers.

@ger21 described exactly how I would carry out the project with one exception, and that is the choice of router bit. I would also heed the advice of @gdonham1 and use a high feed rate in order to avoid melting. My choice of router bit is based on the width of the already existing grooves of which @Martyc wants to round the edges. A point cutting roundover bit would not apply a full 90° roundover in a single pass while an ovolo, a.k.a plunge roundover with a 1/4 inch small diameter would.
While I suggested a Freud ovolo bit, I could not find a Freud point cutting roundover bit!

The two .pdf files linked are exactly the same images as the .png which show the images directly.